A paper published in Nature on Sept. 1, 2025 studied the mtDNA of the American mastodon, Mammut americanum, and concluded that they arrived in different waves to America from Asia, and that they moved about North America as the ice sheets came and went.
Paper: Karpinski, E., Hackenberger, D., Zazula, G. et al. American mastodon mitochondrial genomes suggest multiple dispersal events in response to Pleistocene climate oscillations. Nat Commun 11, 4048 (2020). https://doi.org/10.1038/s41467-020-17893-z
They went and came with the advancing and retreating glaciers: "American mastodons only occupied higher latitudes (i.e., Canada and Alaska) during interglacials, when prevailing warm climatic conditions supported the establishment of forests and wetlands." When the ice sheets advanced, they retreated southwards, away from the polar regions.
The authors wonder why this happened time and time again, but after the last ice age, it didn't: " However, this pattern also poses further questions: for example, why were species that had managed to repeatedly expand into the northernmost parts of North America during previous interglacials unable to do so following the return to interglacial conditions after the last glacial maximum (~21 kya)? Were they already in severe decline?
The answer is obvious, by the last Ice Age 21 kya, human beings were well established in America, wiping out the megafauna. So this ended the pattern.
If mammoths managed to cope with the harsh polar environment and move into America, humans could have done the same, during any of the low-sea-level periods at the peak of each ice age. Below is an image from this paper showing when those windows of opportunity opened.
The caption for this image reads: "a Global stack of benthic foraminifera δ18O for the last 1 million years, which tracks changes in deep-water temperature and global ice volume. The y-axis has been inverted so that periods of low ice buildup (and higher temperatures—red) are at the top of the graph, and periods of greater ice buildup (and lower temperatures—blue) are at the bottom. Marine Isotope Stage (MIS) extents are indicated with black bars above (interglacials) or below (glacials) the δ18O record." We marked with red arrows the periods where a "bridge" would have formed in Beringia. See how many!
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